Raman Studies of Solution Polyglycine Conformations

Polyglycine (polygly) is an important model system for understanding the structural preferences of unfolded polypeptides in solution. We utilized UV resonance and visible Raman spectroscopy to investigate the conformational preferences of polygly peptides of different lengths in water containing LiCl and LiClO4. Lithium salts increase the solubility of polygly. Our study indicates that in solution the conformational ensemble of polygly, as well as central peptide bonds of gly(5) and gly(6), are dominated by the 3, extended helix, also known as the polyglycine II conformation (PGII). This preference of the polygly backbone for the PGII conformation in solution is likely a result of favorable interactions between carbonyl dipoles in these extended helices. We found that high concentrations of Li+ stabilize the PGII conformation in solution, most likely by polarizing the peptide bond carbonyls that makes PGII-stabilizing carbonyl-carbonyl electrostatic interactions more favorable. This ability of Li+ to stabilize 3(1)-helix conformations in solution gives use to the denaturing ability of lithium salts.